1. Field of the Invention
The present invention relates to a touch panel system and an electronic device including the touch panel system. Particularly, the present invention relates to a touch panel system and an electronic device each of which is capable of reliably and effectively removing (canceling) a noise generated by a display device, etc.
2. Description of the Related Art
Recently, introduction of touch panel systems to various kinds of electronic devices has been growing rapidly. For example, the touch panel systems are introduced to portable information devices such as smartphones and automatic vending machines such as automatic ticket machines.
The touch panel system is typically configured to include (i) a display device and (ii) a touch panel stacked on an upper side (front surface) of the display device. Therefore, a sensor provided on the touch panel is likely to be affected not only by a noise such as a clock generated in the display device but also by other noises coming from the outside. Such the noises lead to impairment in detection sensitivity for a touch operation.
Japanese Unexamined Patent Application Publication No. 2001-125744 (published on May 11, 2001, and referred to as a “publicly known document 1” hereinafter) describes a touch panel system (coordinates input device) including a countermeasure against such noises. The touch panel system of the publicly known document 1 includes a noise processing section for removing a noise. FIG. 19 is a block diagram illustrating a noise processing section 100 included in the touch panel system of the publicly known document 1. As shown in FIG. 19, the noise processing section 100 includes a filter section 101, a logical inversion section 102, and an adding section 103. The filter section 101 receives an output signal (analog signal) from a sensor provided in a touch panel (not illustrated). The filter section 101 extracts, as a noise signal, an AC signal component included in the input signal. The logical inversion section 102 inverts by 180° the phase of the noise signal thus extracted. The adding section 103 adds, to the input signal which is supplied to the filter section 101 and which includes the noise signal, the noise signal whose phase has been inverted by 180°.
Thus, according to the touch panel system of the publicly known document 1, the noise signal extracted by the filter section 101 is inverted, and the signal thus inverted is added to the input signal (analog signal) supplied from the sensor. Namely, to the noise component included in the input signal supplied from the sensor, such a signal is added which has the same level as the noise component and whose phase has been inverted. This cancels the noise superimposed on the input signal supplied from the sensor. This makes it possible to reduce effects given by the noise included in the input signal supplied from the sensor.
However, the touch panel system of the publicly known document 1 has a problem of being incapable of removing noises other than an AC signal component.
Specifically, as described above, with respect to an input signal supplied from the sensor, the touch panel system of the publicly known document 1 regards as a noise an AC signal component included in the input signal. The filter section 101 extracts the AC signal, and thereafter the logical inversion section 102 inverts the phase of the AC signal by 180°. Further, the adding section 103 adds the inverted signal to the input signal which includes the AC signal component. Thus, for the noise processing according to the publicly known document 1, the process performed by the filter section 101 for extracting the AC signal component is the most important.
However, the publicly known document 1 fails to disclose details of the configuration of the filter section 101. Therefore, it is unknown how much noise the touch panel system of the publicly known document 1 can remove. Furthermore, the publicly known document 1 regards as a noise an AC signal component included in an analog signal. Namely, the touch panel system of the publicly known document 1 basically assumes removal of an impulse noise only, and does not assume, as the subject of removal, noises other than the impulse noise. Therefore, the touch panel system of the publicly known document 1 cannot reliably cancel a wide variety of noises other than the impulse noise.
Furthermore, it is desired to reduce power consumption and enhance detection sensitivity in a touch panel system.